1. Field of the Invention
The present invention relates to microelectronic device packaging. More particularly, the present invention relates to underfill packaging of a flip-chip package. In particular, the present invention relates to a vacuum-assisted underfill process.
2. Description of Related Art
As the process of miniaturization progresses, chip packaging is also required to be miniaturized. This requirement has resulted in chip-scale packaging, wherein the ultimate goal is to have the chip and the chip package be virtually the same size.
A chip package usually includes a mounting substrate and a semiconductor chip or die that is located on or in the mounting substrate. One class of chip packaging includes semiconductor chips that are produced with C4 (controlled collapse chip connect) solder connections, on an active surface thereof, for purposes of electrically contacting the integrated circuit on the active surface of the chip to contact pads on the mounting substrate. The term active surface of a semiconductor chip or die, as used herein, means the surface of the chip or die which carries integrated circuitry. The term back surface, as used herein, means a side of the semiconductor chip or die that is opposite and parallel planar with the active surface.
During chip packaging, an epoxy is introduced under capillary action into a space between the die and the mounting substrate and is cured thereafter. The epoxy acts to bond the die to the mounting substrate and to protect the C4 solder connections during the temperature cycling it will experience during the product""s lifetime.
As depicted in FIG. 1A and FIG. 1B, a chip package 10 includes a die 12, a mounting substrate 14, an electrical connection 16 such as a C4 bump, and an underfill material 18 that has been inserted between the die 12 and the mounting substrate 14 by capillary action. FIG. 1A depicts a nonuniform profile 20 and 21 of the underfill material 18 including a fillet portion 22 and 23, and an interstitial portion 24 that is sandwiched between the die 12 and the mounting substrate 14. It is noted in FIG. 1B that there is a tongue 25 of epoxy underfill material 18 on one side thereof. Accordingly, the fillet portion 22 and 23 that is depicted at profiles 20 and 21 in FIG. 1A exhibits an asymmetrical footprint upon the mounting substrate 14 as depicted in FIG. 1B. Although this tongue 25 of epoxy underfill material 18 may be of no consequence in some prior art embodiments, the pressure to miniaturize and to get even tighter bump pitch and chip-to-package gap height causes the presence of the tongue 25 to be undesirable.
One possible solution that is used in production is depicted in FIG. 2A and FIG. 2B. Processing is accomplished by directing a mold press 26 against a die 12 and a mounting substrate 14. Between the mold press 26 and the die 12 and mounting substrate 14, an adhesion-resistant film 28 is placed that is stretched and held while an underfill material feed tube 30 and a vent or vacuum tube 32 are used to flow underfill material 18 between die 12 and mounting substrate 14. After the underfill material 18 has been properly flowed therebetween to form uniform fillet portions 22 and the interstitial portion 24, the adhesion resistant film 28 and the mold press 26 are removed as depicted in FIG. 2B. As the adhesion resistant film 28 and the mold press 26 are removed, some wicking action between the adhesion resistant film 28 and the underfill material 18 forms an uneven surface 34 (depicted in an arbitrary shape and surface roughness) that often must be smoothed after curing. Additionally, and more serious to process yield, some wick spillage 36 forms on the back surface 38 of die 12 that must be removed.